1. Field of Invention
The invention relates to a static random access memory (SRAM) cell and, in particular, to a non-volatile SRAM cell.
2. Related Art
A digital system usually needs to constantly read and store digital data during operations. Therefore, memory units with the function of keeping data are important elements in realizing a digital system. They can be categorized into the following types: the random access memory (RAM), the serial access memory (SAM), and the content access memory (CAM).
The semiconductor memory is normally a matrix comprised of storage cells. Each cell can store one bit of information. When necessary, data can be arbitrarily stored in or read out from each cell. Therefore, this type of memory is called the random access memory (RAM) to be distinguished from the read only memory (ROM). A main advantage of the RAM is that the access time of each bit in the matrix is the same. However, the drawback is that when the power is turned off, all data will be lost. This phenomenon is described as volatile. That is, when the power is turned off, the data stored in the RAM disappear immediately. On the other hand, the data stored in the ROM can be kept forever and do not disappear with the power shutdown. Consequently, the ROM is also called non-volatile memory. Therefore, according to the storage properties, the memory can be volatile or non-volatile. The biggest difference is that the data stored in the non-volatile memory can be kept even after the power is turned off. Non-volatile memory includes read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electric erasable programmable read only memory (EEPROM), and flash memory. The ROM only allows data to be written in and the data cannot be modified. The EPROM requires the use of ultraviolet (UV) light to modify the data. The EEPROM modifies data using voltage changes.
Since the RAM cannot continuously keep its data, a non-volatile SRAM (nvSRAM) is proposed in the prior art to deal with this intrinsic limitation of the RAM. Some hand-held or portable digital products use the battery as its power supply. Once the battery cannot keep supplying power and no backup power source is available, data stored in the memory may be lost. In this case, nvSRAM is an ideal solution. Such an nvSRAM has two parts: one being a static random access memory (SRAM) unit and the other being a non-volatile memory unit. The SRAM part is used to temporarily store data when power is provided. The non-volatile memory unit is used to stored data when the power is discontinued and to restore the data to the SRAM once the power supply is resumed.
An important factor in the research and development of this kind of nvSRAM is that data have to be immediately stored in the non-volatile memory unit before the power completely disappear. Therefore, an nvSRAM that has a rapid storage capability is disclosed. For example, the U.S. Pat. No. 6,097,329, “Non-volatile Static Random Access Memory with High Speed STORE Capability,” uses an SRAM cell and an nv cell to make the nvSRAM. A controller is further employed to store data to achieve the goal of high-speed access.
There are already many solutions for the nvSRAM in the prior art. The invention provides a new nvSRAM cell that uses fewer transistors but achieves a faster access speed.